Image: Atlas Copco
Rotary screw air compressors are the most widely used air compressor models in the industrial marketplace. Rotary screw compressors provide continuous compressed air for precision tasks and they are extremely efficient and remarkably quiet. Screw compressors utilize some of the most advanced technology available in the compressor industry and have a very low energy consumption. These machines are built to run 24 hours a day, 7 days a week, for many years!
There are many different types of rotary screw compressors. The classifications vary based on stages, cooling method, drive type, among others.The most widely used type is the single stage, oil-injected rotary screw compressor.The main difference is that oil-lubricated compressors inject oil into the compression chamber to serve a few different purposes.
Oil-free compressors can be used in specific applications where the tiny amounts of excess oil in the air would contaminate the product or process.
Oil-injected models are by far the most common and for the purpose of this article we will focus on this type.
The power (and name) behind these remarkable machines comes from two counter-rotating screws which are housed in a chamber, formally known as an airend. Outside air first travels through a filter to catch any harmful particles and debris that could cause damage. Once filtered, the air goes through an inlet valve and into the space between the interlocking screws. As the rotors turn, the air moves along and travels to the other end of the compression chamber.
The area in which the air is contained gets increasingly smaller as the air moves along, and the smaller space increases the pressure. This process is one smooth continuous motion, and therefore produces minimal pulsing or surging which can occur with piston compressors. The result is a high-volume, steady stream of compressed air.
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In an oil-injected rotary screw air compressor, the oil serves five key purposes: it cleans, cools, lubricates, seals, and protects. Therefore, what is produced by the airend is a compressed air/oil mixture. This mixture flows into a separator tank where the two are, well… separated! Primarily, a mechanical separation takes place using a filter. Further, oil can be removed using centrifugal force or a rapid change in direction. In the case of centrifugal force, rapid spinning allows the heavier oil particles to drop to the bottom while the lighter air spins around on top. Simultaneously, the separated elements leave the tank.
The air leaves the tank and travels through a minimum-pressure valve, and (in some cases) a cooler. The minimum-pressure check valve is dual-purpose and ensures there is sufficient air pressure for proper operation while also working to keep the air pressure from coming back into the system so, when necessary, the motor can start without a load.
The compressed air can still be very hot and might need to go through a cooler to bring it down to 12-20 degrees Fahrenheit above ambient temperature. This cooling process causes water vapor to condense inside the cooler, and therefore, in most compressors there is a water separation stage before the air leaves the package. After these last stages, compressed air finally leaves the compressor ready for treatment, regulation, storage, and to the customer’s processes.
Meanwhile, as the oil was collected from the tank earlier, a line transports the oil back to the compression chamber to be used again. But first, it must be filtered and possibly cooled. A thermostatic valve determines if the oil is too hot and, if necessary, sends the oil through an oil cooler before being filtered. Once filtered, the oil is ready to begin the journey over again.
Check out this cool animation from Atlas Copco detailing the process in an oil-injected rotary screw compressor:
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